When metal fine particles are irradiated with light, a resonance absorption phenomenon, which is so-called plasmon absorption, occurs. In the resonance absorption phenomenon, an absorption wavelength differs depending on the kind and shape of metal. For example, a gold colloid, in which spherical gold fine particles are dispersed in water, has an absorption region around 530 nm. In contrast, it is well known that in the case of gold fine particles in a rod shape having a length in a short axis of about 10 nm, they have an absorption at a long wavelength side, which is attributed to a long axis of the rods, in addition to the absorption at 530 nm, which is attributed to the short axis of the rods (S-S. Chang et al. Langmuir, 1999, 15th issue, pages 701 to 709).
It has been suggested that a resin composition containing noble metal fine particles as a coating or pigment can be used for optical materials such as an optical filter (Japanese Patent Application, First Publication No. H11-80647). As materials used for an optical filter, a color filter containing a dye having a specific chemical structure (Japanese Patent Application, First Publication No. 2001-108815), and an optical filter comprising a coating containing both a dye having a specific chemical structure and a metal complex (Japanese Patent Application, First Publication No. 2002-22935) are known. The former color filter comprises a striped pattern of three colors of red, green and blue on a transparent substrate. The latter optical filter has a light transmissivity of 0.01 to 30% in a wavelength range from 750 to 1,100 nm.
A production method for producing metal fine patterns using plasmon excitation of metal fine particles is also known (Japanese Patent Application, First Publication No. 2001-64794). This production method is a method using a phenomenon whereby when metal fine particles are supported on the flat surface of a semiconductor or solid metal, metal fine particles linearly elongate due to plasmon excitation.
As explained above, it is known that noble fine particles can be used for a coating or pigment for resin compositions; however, the noble fine particles used are spherical. For example, plasmon color, which spherical gold fine particles generate, is limited to visible light such as blue, bluish-purple, and reddish-purple. Therefore, a composition which uses plasmon absorption of spherical gold fine particles, or a substrate which is obtained by coating or kneading the composition, only has tones such as blue, bluish-purple, and reddish-purple.
Many color filters containing a dye having a specific chemical structure discolor or decrease their absorption capacity when a long time has passed, because the dye has inferior heat resistance, light resistance, and chemical resistance compared with pigments and metal fine particles, and they have problems in reliability. In a method in which metal fine particles are made to grow on the surface of a solid, the metal fine particles are supported and grown on the surface of the solid. Therefore, it is impossible to disperse the metal fine particles in many solvents, binders, and production of a coating is difficult. In addition, in Patent Publications, plasmon absorption of metal fine particles is used only to grow the metal fine particles during synthesis steps, and selective light absorption characteristics at a specific wavelength due to a long direction of the metal fine particles is not used.
The present invention solves the problems of conventional color filters or the fine linear elongation method of metal fine particles, and yields tone, which is not yielded by conventional spherical metal fine particles, by making metal fine particles into a rod shape having an aspect ratio of 1.1 or greater, that is, metal nano-rods. In addition, the present invention provides a pigment having excellent wavelength absorption properties and heat resistance, and the like, and metal fine particles that can be suitably used for an optical filter material.
The metal nano-rods are expected to be used for a variety of applications such as optical filter materials, near infrared light absorbing materials, pigments, and cosmetics, which use light absorption properties of the metal nano-rods, and conductive materials, wiring materials, electromagnetic interference shielding materials which use conductivity of the metal nano-rods.
As a production method for producing metal nano-rods, an electrochemical reduction method, a chemical reduction method, a photoreduction method, an ultrasonic wave irradiation method, and the like are known.
The electrochemical reduction method is a method in which gold fine particles are obtained by putting a gold-plated anode and a platinum-plated cathode in an electrolyte containing a surfactant, and passing a constant current between the anode and cathode. Gold ions generated at the anode are reduced at the cathode, and become gold fine particles. Then, the gold fine particles grow into a rod shape due to the functions of the surfactant, and become gold nano-rods in nano-size. In the electrolytic method, a silver plate is immersed in an electrolyte. It is reported that the area of the immersed silver plate in the electrolyte affects the length of the rods (Langmuir, 1999, 15th issue, pages 701 to 709). However, the amount of eluted silver and elution rate change depending on surface conditions of the silver plate. Therefore, it is difficult to adjust the area of the immersed silver plate, and control sufficiently the aspect ratio of the metal nano-rods.
The chemical reduction method is a method in which gold nano-rods are obtained by adding a reducing agent to an aqueous solution of chloroauric acid, reducing chloroauric acid, growing gold nano-particles to obtain “seed particles”, transferring the seed particles to an aqueous solution of chloroauric acid, and growing the seed particles in the aqueous solution to obtain gold nano-rods (J. Phys. Chem. B, 2001, 105th issue, pages 4065 to 4067). In this method, it is possible to control the length of the obtained rods by changing the amount of seed particles which are transferred to a growth reaction vessel. However, a reaction vessel for obtaining the seed particles and a reaction vessel for growing the seed particles are necessary, and production processes are complicated and troublesome.
The photoreduction method is a method in which gold fine particles are obtained by irradiating ultraviolet light to an aqueous solution of chloroauric acid, and reducing chloroauric acid in the solution (J. Am. Chem. Soc. 2002, 124th issue, pages 14,316 to 14,318). This method does not need two vessels, which are required in the chemical reduction method, and the length of the rods can be controlled by adjusting the irradiation time. However, this method has a problem in that the reaction time is long.
The present invention solves these problems of conventional production methods for producing metal fine particles. The present invention provides a production method which can easily control the aspect ratio of metal fine particles, and compositions containing the metal fine particles produced by the production method.
In addition, the present invention further provides preferable usages of the metal fine particle compositions.